Whether it be for a new website design or just a placeholder in a current project, stock photos are normally a necessity. They are useful and make the design look better overall if the photo is professional. From minimalist photos to epic landscapes and self portraits, you can find just about anything on these great free stock photography sites.
Compfight is probably my #1 favorite. Instead of it's own content, it sorts through all of the photos on flickr allowing you to search by commercial free, creative commons, and other awesome parameters. You can find photos for anything being that flickr has millions of great, high quality photos.
Stock Exchange is a great place for finding any photo. It has been around a long time so it has the biggest selection. Granted some of the quality is lacking, but I normally don't search too long before finding a good photo!
Unsplash is a relatively new site that was created by ooomf. Unsplash shares 10 new photo ever 10 days. These are some of the highest quality stock photos around, and fit perfectly into designs, posts, and other mockups.
Pixabay has thousands of free to use images. This site is a little less reliable for commercial free images, but if you only need images for personal projects, it will be perfect for you.
OpenPhoto is a great resource for free photos that has been around for over 10 years. It has an insanely large collection and an easy to use sort feature by tag. The quality isn't as amazing as the sources above, but it just takes a bit longer searching to find the right photos.
Aside from the obvious stock resource sites, keep in mind that there are plenty of other options. Some artists and designs allow for use of their digital illustrations or designs. Just be sure to ask for permission first. Also try taking your own photos, you can do much better that you think. Check out our photography section for some inspiration.